The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 to Stemme.                There are various known types of thermal inkjet (Bubblejet™ is owned by Canon K.K.) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate which is mechanically secured to the walls of the chamber. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles.        
Thermal inkjet printheads are traditionally prone to overheating. The rapid successive vaporization of ink during printing can build up heat in the printhead. If too much builds up in the printhead, the ink will boil in an uncontrolled manner. This heat is removed from the printhead either by an active cooling system or with heats sinks and the use of small nozzle arrays. The overheating problem has limited the firing frequency of the nozzles and printhead size, both of which reduce the print speed.
The Applicant has developed a range of pagewidth printheads that overcome the problem of excess heat generation. The large pagewidth arrays and high nozzle firing frequencies provide print speeds in excess of 60 pages per minute at full color 1600 dpi resolution. These printheads avoid excess heat generation by reducing the energy used by the heaters to eject the drops of ink. The heat input to the printhead by the heaters is removed from the printhead by the ejected drops of ink.
One aspect of reducing the energy required to eject drops of ink is a reduction in the mass of the ejected drop, and hence the volume of the drop. The Applicant's ‘self cooling’ printheads eject drops of about 1 pl to 2 pl (pico-liters). Unfortunately drop volumes this small are susceptible to trajectory misdirection. The trajectory of the ejected drop is particularly sensitive to the nozzle geometry and the shape of the bubble generated by the heater element. It will be appreciated that any misdirection of the ejected ink drops is detrimental to print quality.
Fluidic symmetry around the heater is not possible unless the heater is suspended directly over the ink inlet. The Applicant has developed printheads with this arrangement (see U.S. Pat. No. 6,755,509 filed Nov. 23, 2002), however there are production efficiencies and nozzle density gains available if multiple ink chambers are supplied from a single ink supply channel through the supporting wafer. This requires that the individual chambers are supplied with ink through lateral inlets—that is, inlets extending parallel to the planes of the heaters and the nozzles. As the heater is laterally bounded by the chamber walls except for the ink inlet, the bubble generated by the heater is distorted by this asymmetry. The inlet can be lengthened and or narrowed to increase its fluidic resistance to back flow caused by the bubble. This will reduce the fluidic asymmetry caused by the inlet but also increase the chamber refill times because of the higher flow resistance.